Jewels of the Southern Cross

Press, 30 December 1976, Page 8

Jewels of the Southern Cross

Astronomy EM. Bateson |

Ihe five brightest stars of the Southern Cross — Alpha, Beta, Gamma, Delta and Epsilon — when taken in a clockwise direction around the constallation are also arranged in order of decreasing brightness. If we take a line from Gamma to Beta Crucis and extend it slightly beyond Beta, we will find a very tine open cluster. This is variously known as the Kappa Crucis cluster, the Jewel cluster or the Jewel Box. Astronomers know it as NGC 4755 from its catalogue number. The name of the Jewel Box was bestowed on the cluster by Sir John Herschel, who observed the southern skies from South Africa. He compared it to a sparkling selection of jewels. This description is apt because of> the vivid and contrasting colours of the individual stars. There are just over 200 stars in this cluster, although it is probable that there are many more fainter members. It has proved difficult to separate these from faint foreground stars. Ihe latest investigation places the cluster at a distance of about 7500 light ' ears, which is more distant than earlier researches indicated. Its linear diameter is 23 light years.

The principal stars form an A-shaped pattern with a deep red star in the centre. The stars differ widely in luminosity. the cluster is dominated by five supergiant stars, one of which is red and the others bluish. It is unusual to find an open cluster containing different types of supergiants. Such objects are very luminous and large. The red supergiant, for instance, is fully 25.0(10 times as bright intrinsically as our Sun. Its diameter probably exceeds that of the Sun bv about 100 times. Ihe age of the cluster has been placed at about “t’M years, which means that it is comparatively young. In determining the age of a cluster it is assumed that the individual stars condensed out ot a large cloud of matter at about the same time. Ihe time from when the first star in a cluster appeared until the last one was born nav cover a few million years. That is however, a very short interval on cosmic time scales. If the surface temperature of the stars are plotted against their absolute luminosity we find that the majority fall along a line running front the lower right to the upper lett. This line is called the Main Sequence. The stars which are cool and faint are clustered towards the lower right of our diagram. whilst the hot, bright stars are found towards the upper left. These latter are the most massive and have very short life times. I'he Jewel Box contains stars covering the complete range of stellar masses. When a diagram as we have described is made for this cluster it is possible to determine its age from the position of the more massive stars.

Such diagrams are called H-R diagrams from the names of the two astronomers who first de\ eloped this powerful tool. They were Hertzsprung and Russell.

l ying beside a line from Alpha to Beta Crucis, and just south of the !• ftox, is a pear-shaped dark aiea in which only one

star can oe seen with the naked eye. This is the Coalsack, sometimes called the Sootbag. It appears dark because a great cloud of dust and gas lies between us and the stars beyond.

Long exposure photographs show that there are, on the average, onethird as many faint stars

in the Coalsack as in an adjacent area of the same size. The dark matter is not uniform in distribution but arranged more in clumps of matter.

A number of globulelike units have been found within the region. These max ultimately become

protostars — stars in the making — as the irregularities collapse.

Mixed with the gas of such clouds are cosmic grains of dust, which some astronomers suggest are composed of graphite. The temperature of these grains is very low’, probably of the order of a few degrees. The Coalsack was first observed by Europeans in 1499, although known to southern peoples long before that. The Australian aborigines noted that it formed part of connected dark patches that stretched through the Milky Wav

from the Southern Cross to Scorpius and Ophiuchus. They' likened it to the Emu, with the Coalsack representing the head and sharp beak, and the narrow dark band from Centaurus to Norma the long thin neck. The dark clouds of Scorpius and Sagittarius they regarded as the body with the legs formed by the dark lanes in Ophiuchus. They regarded the Emu as lying at the foot of a tree, represented by the stars of the Cross, waiting for an opossum that had taken refuge in its branches.

It is interesting to recall that the Southern Cross was visible on the horizon of Jerusalem about the time that Christ was crucified. Some 3000 years before that all its stars were visible from much further north. The reason why' this constellation was at one time visible from the far north is that precession of the Earth’s axis causes the celestial poles to shift slowly with respect to the stars. The Earth’ is like a spinning top, rotating rapidly about its axis, while the axis revolves slowly about the vertical. This produces a slow

change of direction uf me axis, due to the pull of the Sun and Moon on the Earth’s equatorial bulge. As a result the axis of rotation revolves in a cone, some 47deg. in diameter, once every 26,000 years. JANUARY PLANETS January skies wil! see Venus increase in brightness to be very conspicuous in the evening sky where it reaches its greatest elongation :ast of the Sun on January 25.

Jupiter crossing the meridian to the north, will also be prominent, although it is some two magnitudes fainter than Venus.

Saturn, in Cancer, is nearing opposition which occurs on February 2 and in our latitudes will be visible in the north-east when twilight ends.

Mercury and Mars appear in the morning sky from mid-January but will be too close to the Sun for satisfactory viewing until next month. The Moon will be close to Jupiter on January 1 in the early evening and again at 11 p.m. on Unitary 28.

Venus and the Moon will present an attractive appearance together on the evening of January 23 in the western sky.